This book is a plain account of the development of the neverfailing resource of the embryologist, the chick. It has been necessary to fill certain gaps in our knowledge of the development of the chick by descriptions of other birds. But the account does not go beyond the class Aves, and it applies exclusively to the chick except where there is specific statement to the contrary. Projected chapters on the integument, muscular system, physiology of development, teratology, and history of the subject have been omitted, as the book seemed to be already sufficiently long. The account has been written directly from the material in almost every part, and it has involved some special investigations, particularly on the early development undertaken by Doctor Mary Blount and Doctor J. T. Patterson, to whom acknowledgments are due for permission to incorporate their results before full publication by the authors. As the book is meant for the use of beginners in embryology, references to authors are usually omitted except where the account is based directly on the description of a single investigator. A fairly full list of original sources is published as an appendix.

−

Figures borrowed from other publications are credited in the legends to the figures. The majority of the illustrations are from original preparations of the author: Figures 46, 48, 50, 51, 52, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 71, 72, 73, 74, 75, 99, 105 and 106 were drawn by Mr. K. Hayashi; the remainder of the original drawings were executed by Mr. Kenji Toda. The photographs in Figures 118, 119, 120, 168, 181, 182, 189, 194, 197, and 231 are the work of Mr. Willard C. Green. Some of the figures may be studied with advantage for points not described in the text.

+

Professor In The University Of Chicago

−

Acknowledgments are also due my colleague, Professor W. L. Tower for much assistance, and to Doctor Rov L. Moodie for special work on the skeleton, and photographs of potash preparations reproduced in Figures 242, 246, 249 and 250.

+

Second Edition, Revised

−

The best introduction to the problems opened up by the study of embryology is a careful first-hand study of some one species. It is in this sense that the book may serve as an introduction to embryology, if its study is accompanied by careful laboratory work. In some respects it is fuller, and in others less complete, than other books with which it might be compared. On its comparative and experimental sides, embryology is the only key to the solution of some of the most fundamental problems of biology. The fact that comparative and experimental embryology receive bare mention is not due to any lack of appreciation of their interest and importance, but to the conviction that the beginner is not prepared to appreciate these problems at the start; to the belief that our teachers of embryology are competent to remedy omissions; and finally to the circumstance that no one book can, as a matter of fact, cover the entire field, except in the most superficial way.

+

New York

+

Henry Holt And Company

−

The development before laying and the first three days of incubation are treated by stages as far as possible, and this matter constitutes Part I of the book. It involves the study of the origin of the primordia of most of the organs. The matter concerning the later development is classified by the organs concerned, which seems to be the only possible way, and this constitutes Part II. The first part is complete in itself, so far as it goes, and no doubt it will be the only part consulted by some students.

+

1919

−

The attempt to present a consecutive account of the development of the form on which so many classics in the history of embryology have been based is no slight undertaking. The author can hardly hope that he has avoided omissions and errors, and he will be sincerely grateful to those who call such to his attention.

+

Copyright, 1908, 1919,

−

==Contents==

+

By Henry Holt And Company

−

Ixtroduction

−

I. The Cell Theory . 1

+

==Preface to First Edition==

+

This book is a plain account of the development of the neverfailing resource of the embryologist, the chick. It has been necessary to fill certain gaps in our knowledge of the development of the chick by descriptions of other birds. But the account does not go beyond the class Aves, and it applies exclusively to the chick except where there is specific statement to the contrary. Projected chapters on the integument, muscular system, physiology of development, teratology, and history of the subject have been omitted, as the book seemed to be already sufficiently long. The account has been written directly from the material in almost every part, and it has involved some special investigations, particularly on the early development undertaken by Doctor Mary Blount and Doctor J. T. Patterson, to whom acknowledgments are due for permission to incorporate their results before full publication by the authors. As the book is meant for the use of beginners in embryology, references to authors are usually omitted except where the account is based directly on the description of a single investigator. A fairly full list of original sources is published as an appendix.

−

II. The Recapitulation Theory 3

+

Figures borrowed from other publications are credited in the legends to the figures. The majority of the illustrations are from original preparations of the author: Figures 46, 48, 50, 51, 52, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 71, 72, 73, 74, 75, 99, 105 and 106 were drawn by Mr. K. Hayashi; the remainder of the original drawings were executed by Mr. Kenji Toda. The photographs in Figures 118, 119, 120, 168, 181, 182, 189, 194, 197, and 231 are the work of Mr. Willard C. Green. Some of the figures may be studied with advantage for points not described in the text.

−

III. The Physiology of Development 6

+

Acknowledgments are also due my colleague, Professor W. L. Tower for much assistance, and to Doctor Rov L. Moodie for special work on the skeleton, and photographs of potash preparations reproduced in Figures 242, 246, 249 and 250.

−

IV. Embryonic Primordia and the Law of Genetic Restric tion 8

+

The best introduction to the problems opened up by the study of embryology is a careful first-hand study of some one species. It is in this sense that the book may serve as an introduction to embryology, if its study is accompanied by careful laboratory work. In some respects it is fuller, and in others less complete, than other books with which it might be compared. On its comparative and experimental sides, embryology is the only key to the solution of some of the most fundamental problems of biology. The fact that comparative and experimental embryology receive bare mention is not due to any lack of appreciation of their interest and importance, but to the conviction that the beginner is not prepared to appreciate these problems at the start; to the belief that our teachers of embryology are competent to remedy omissions; and finally to the circumstance that no one book can, as a matter of fact, cover the entire field, except in the most superficial way.

−

V. General Characters of Germ-cells 9

+

The development before laying and the first three days of incubation are treated by stages as far as possible, and this matter constitutes Part I of the book. It involves the study of the origin of the primordia of most of the organs. The matter concerning the later development is classified by the organs concerned, which seems to be the only possible way, and this constitutes Part II. The first part is complete in itself, so far as it goes, and no doubt it will be the only part consulted by some students.

−

The Spermatozoon 9

+

The attempt to present a consecutive account of the development of the form on which so many classics in the history of embryology have been based is no slight undertaking. The author can hardly hope that he has avoided omissions and errors, and he will be sincerely grateful to those who call such to his attention.

−

The Ovum 10

−

Comparison of the Germ-cells 12

+

{{Review - Lillie’s Development of the Chicken collapsetable}}

−

VI. Polarity and Organization of the Ovum .... 14

+

==Contents==

−

−

Part I The Early Development To The End Of The Third Day

−

−

CHAPTER I. THE EGG 17

−

−

Chemical Composition of the Hen's Egg 20

−

−

Formation of the Egg 21

−

−

Abnormal Eggs 25

−

−

Ovogenesis 26

−

−

CHAPTER II. THE DEVELOPMENT PRIOR TO LAYING 32

−

−

I. Maturation 32

−

−

11. Fertilization 35

−

−

III. Cleavage of the Ovum 38

−

−

The Hen's Egg 39

−

−

The Pigeon's Egg 43

−

−

IV. Origin of the Periblastic Nuclei, Formation of the

−

−

Germ-wall 47

−

−

V. Origin of the Ectoderm and Entoderm ...... 52

−

−

CHAPTER III. OUTLINE OF DEVELOPMENT, ORIENTATION, CHRONOLOGY 61

−

−

Orientation 63

−

−

Chronology {Classification of Stages) 64

−

−

Tables of the Developyyient of the Chick 68

−

−

CHAPTER IV. FROM LAYIXG TO THE FORMATIOX OF

−

−

THE FIRST SOMITE 69

−

−

I. Structure of the Unincubated Blastoderm .... 69

−

−

II. The Primitive Streak 69

−

−

Total Views 69

−

−

Sections 74

−

−

The Head-process 80

−

−

hiterpretation of the Primitive Streak 83

−

−

III. The Mesoderm of the Opaque Area 86

−

−

IV. The Germ-wall 90

−

−

CHAPTER V. HEAD-FOLD TO TWELVE SOMITES

−

−

(From about the twenty-first to the thirty-third hour of incubation) 91

−

−

I. Origin of the Head-fold 91

−

−

II. Formation of the Fore-gut 93

−

−

III. Origin of the Xeural Tube 95

−

−

The Medullary Plate 95

−

−

The Neural Groove and Folds 97

−

−

Primary Divisions of the Neural Tube 105

−

−

Origin of the Primary Divisions of the Embryonic Brain 108

−

−

IV. The Mesoblast 109

−

−

Primary Structure of the Sornites 11-4

−

−

The Nephrotome, or Intermediate Cell-mass (Middle

−

−

Plate) 114

−

−

The Lateral Plate 115

−

−

Development of the Body-cavity or Cadome 115

−

−

Mesoblast of the Head 116

−

−

Vascular System 117

−

−

Origin of the Heart 119

−

−

The Embryonic Blood-vessels 121

−

−

V. Description of an Embryo with 10 Somites .... 122

−

−

The Nervous System 124

−

−

Alimentary Canal 126

−

−

Vascular System 126

−

−

General 127

−

−

Zones of the Blastoderm 127

−

−

CHAPTER VI. FROM TWELVE TO THIRTY-SIX SOMITES.

−

−

THIRTY-FOUR TO SEVEXTY-TWO HOURS . 130 I. Development of the External Form, and Turning of the Embryo 130

−

−

Separation of the Embryo from the Blastoderm . . . 130

−

−

The Turning of the Embryo and the Embryonic Flexures 133

−

−

II. Origin of the Embryonic Membranes 135

−

−

Origin of the Amnion and Chorion 135

−

−

The Yolk-sac 143

−

−

Origin of the Allantois 143

−

−

Summary of Later History of the Embryonic Membranes . 145

−

−

III. The Xervous System 147

−

−

The Brain 147

−

−

The Neural Crest and the Cranial and Spinal Ganglia 156

−

−

IV. The Organs of Special Sense (Eye, Ear, X'ose) . 164

−

−

The Eye ^ . 164

−

−

The Auditory Sac 168

−

−

The Nose (Olfactory Pits) 169

−

−

V. The Alimentary Canal and its Appendages . . . 170

−

−

The StomodoEum 173

−

−

The Pharynx and Visceral Arches 173

−

−

(Esophagus and Stomach 179

−

−

The Liver 179

−

−

The Pancreas 181

−

−

The Mid-Gut 181

−

−

Ancd Plate, Hind-gut, Post-anal gut and Allantois 182

−

−

VI. History of the Mesoderm 183

−

−

Somites 183

−

−

The Intermediate Cell-mass 190

−

−

The Vascular System 197

−

−

VII. The Body-cavity and Mesenteries 205

−

−

PART II THE FORRTH DAY TO HATCHING, ORGANOGENY, DEVELOPMENT OF THE ORGANS

−

−

CHAPTER VII. THE EXTERXAL FORM OF THE EMBRYO AXD THE EMBRYONIC :\IEMBRAXES 211

−

−

I. The External Form 211

−

−

General 211

−

−

Head 213

−

−

II. Embryonic Membranes . . . 216

−

−

General 216

−

−

The Allantois 220

−

−

The Yolk-sac 225

−

−

The Amnion 231

−

−

Hatching . . 232

−

−

CHAPTER \TII. THE NERVOUS SYSTEM 233

−

−

I. The Neuroblasts 233

−

−

The Medullary Neuroblasts 233

−

−

The Ganglionic Neuroblasts 236

−

−

II. The Development of the Spinal Cord 239

−

−

Central Canal and Fissures of the Cord 242

−

−

Neuroblasts, Commissures, and Fiber Tracts of the Cord . 244

−

−

III. The Development of the Brain 244

−

−

The Telencephalon 245

−

−

The Diencephalon 249

−

−

The Meseyicephalon 251

−

−

The Metencephalon 251

−

−

The Myelencephalon 252

−

−

Commissures of the Brain 252

−

−

IV. The Peripheral Nervous System . 252

−

−

The Spinal Nerves 252

−

−

The Cranial Nerves 261

−

−

CHAPTER IX. ORGANS OF SPECIAL SENSE .... 271

−

−

I. The Eye 271

−

−

The Optic Cup 271

−

−

The Vitreous Humor 275

−

−

The Lens 276

−

−

Anterior Chamber and Cornea 278

−

−

The Choroid and Sclerotic Coats 279

−

−

The Eyelids and Conjunctival Sac 279

−

−

Choroid Fissure, Pecten and Optic Nerve 281

−

−

II. The Development of the Olfactory Organ . . . 285

−

−

III. The Development of the Ear 288

−

−

Development of the Otocyst and Associated Parts . . . 289 The Development of the Tubo-tyyn panic Cavity, External

−

−

Auditory Meatus and Tympanum 297

−

−

CHAPTER X. THE ALIMENTARY TRACT AND ITS APPENDAGES 301

−

−

I. Mouth and Oral Cavity 301

−

−

Beak and Egg-tooth 302

−

−

The Tongue 305

−

−

Oral Glands 306

−

II. Derivatives of the Embryonic Pharynx 306

+

[[Book - The development of the chick (1919)#Introduction|Introduction]]

+

* I. The Cell Theory

+

* II. The Recapitulation Theory

+

* III. The Physiology of Development

+

* IV. Embryonic Primordia and the Law of Genetic Restric tion

+

* V. General Characters of Germ-cells

+

** The Spermatozoon

+

** The Ovum

+

** Comparison of the Germ-cells

+

* VI. Polarity and Organization of the Ovum

−

Fate of the Visceral Clefts 307

+

'''Part I The Early Development To The End Of The Third Day'''

−

Thyroid 307

+

[[Book - The development of the chick (1919) 1|Chapter I. The Egg]]

+

* Chemical Composition of the Hen's Egg

+

* Formation of the Egg

+

* Abnormal Eggs

+

* Ovogenesis

−

Visceral Pouches • • 307

+

[[Book - The development of the chick (1919) 2|Chapter II. The Development Prior To Laying]]

+

* I. Maturation

+

* II. Fertilization

+

* III. Cleavage of the Ovum

+

** The Hen's Egg

+

** The Pigeon's Egg

+

* IV. Origin of the Periblastic Nuclei, Formation of the Germ-wall

+

* V. Origin of the Ectoderm and Entoderm

−

The Thymus 308

+

[[Book - The development of the chick (1919) 3|Chapter III. Outline Of Development, Orientation, Chronology]]

+

* Orientation

+

* Chronology {Classification of Stages)

+

* Tables of the Development of the Chick

−

Epithelial Vestiges 309

+

[[Book - The development of the chick (1919) 4|Chapter IV. From Layixg To The Formatiox Of The First Somite]]

+

* I. Structure of the Unincubated Blastoderm

+

* II. The Primitive Streak

+

** Total Views

+

** Sections

+

** The Head-process

+

** Interpretation of the Primitive Streak

+

* III. The Mesoderm of the Opaque Area

+

* IV. The Germ-wall

−

The Posthranchial Bodies 309

+

[[Book - The development of the chick (1919) 5|Chapter V. Head-Fold To Twelve Somites]]

+

(From about the twenty-first to the thirty-third hour of incubation)

+

* I. Origin of the Head-fold

+

* II. Formation of the Fore-gut

+

* III. Origin of the Xeural Tube

+

** The Medullary Plate

+

** The Neural Groove and Folds

+

** Primary Divisions of the Neural Tube

+

** Origin of the Primary Divisions of the Embryonic Brain

+

* IV. The Mesoblast

+

** Primary Structure of the Somites

+

** The Nephrotome, or Intermediate Cell-mass (Middle Plate)

+

** The Lateral Plate

+

** Development of the Body-cavity or Coelom

+

** Mesoblast of the Head

+

** Vascular System

+

** Origin of the Heart

+

** The Embryonic Blood-vessels

+

* V. Description of an Embryo with 10 Somites

+

** The Nervous System

+

** Alimentary Canal

+

** Vascular System

+

** General

+

** Zones of the Blastoderm

−

III. The (Esophagus, Stomach and Intestine .... 309

+

[[Book - The development of the chick (1919) 6|Chapter VI. From Twelve To Thirty-Six Somites]]

+

Thirty-Four To Sevexty-Two Hours

−

Oesophagus 312

+

* I. Development of the External Form, and Turning of the Embryo

+

** Separation of the Embryo from the Blastoderm

+

** The Turning of the Embryo and the Embryonic Flexures

+

* II. Origin of the Embryonic Membranes

+

** Origin of the Amnion and Chorion

+

** The Yolk-sac

+

** Origin of the Allantois

+

** Summary of Later History of the Embryonic Membranes

+

* III. The Xervous System

+

** The Brain

+

** The Neural Crest and the Cranial and Spinal Ganglia

+

* IV. The Organs of Special Sense (Eye, Ear, Nose)

+

** The Eye

+

** The Auditory Sac

+

** The Nose (Olfactory Pits)

+

* V. The Alimentary Canal and its Appendages

+

** The Stomodoeum

+

** The Pharynx and Visceral Arches

+

** OEsophagus and Stomach

+

** The Liver

+

** The Pancreas

+

** The Mid-Gut

+

** Anal Plate, Hind-gut, Post-anal gut and Allantois

+

* VI. History of the Mesoderm

+

** Somites

+

** The Intermediate Cell-mass

+

** The Vascular System

+

* VII. The Body-cavity and Mesenteries

−

Stomach 313

+

'''Part II The Forrth Day To Hatching, Organogeny, Development Of The Organs'''

−

Large Intestine, Cloaca, and Anus 314

+

[[Book - The development of the chick (1919) 7|Chapter VII. The Exterxal Form Of The Embryo And The Embryonic Membranes]]

+

* I. The External Form

+

** General

+

** Head

+

* II. Embryonic Membranes

+

** General

+

** The Allantois

+

** The Yolk-sac

+

** The Amnion

+

** Hatching

−

IV. The Development of the Liver and Pancreas , . . 319

+

[[Book - The development of the chick (1919) 8|Chapter VIII. The Nervous System]]

+

* I. The Neuroblasts

+

** The Medullary Neuroblasts

+

** The Ganglionic Neuroblasts

+

* II. The Development of the Spinal Cord

+

** Central Canal and Fissures of the Cord

+

** Neuroblasts, Commissures, and Fiber Tracts of the Cord

+

* III. The Development of the Brain

+

** The Telencephalon

+

** The Diencephalon

+

** The Mesencephalon

+

** The Metencephalon

+

** The Myelencephalon

+

** Commissures of the Brain

+

* IV. The Peripheral Nervous System

+

** The Spinal Nerves

+

** The Cranial Nerves

−

The Liver 319

+

[[Book - The development of the chick (1919) 9|Chapter IX. Organs Of Special Sense]]

+

* I. The Eye

+

** The Optic Cup

+

** The Vitreous Humor

+

** The Lens

+

** Anterior Chamber and Cornea

+

** The Choroid and Sclerotic Coats

+

** The Eyelids and Conjunctival Sac

+

** Choroid Fissure, Pecten and Optic Nerve

+

* II. The Development of the Olfactory Organ

+

* III. The Development of the Ear

+

** Development of the Otocyst and Associated Parts

+

** The Development of the Tubo-tyyn panic Cavity, External Auditory Meatus and Tympanum

−

The Pancreas 323

+

[[Book - The development of the chick (1919) 10|Chapter X. The Alimentary Tract And Its Appendages]]

+

* I. Mouth and Oral Cavity

+

** Beak and Egg-tooth

+

** The Tongue

+

** Oral Glands

+

* II. Derivatives of the Embryonic Pharynx

+

** Fate of the Visceral Clefts

+

** Thyroid

+

** Visceral Pouches

+

** The Thymus

+

** Epithelial Vestiges

+

** The Posthranchial Bodies

+

* III. The (Esophagus, Stomach and Intestine

+

** Oesophagus

+

** Stomach

+

** Large Intestine, Cloaca, and Anus

+

* IV. The Development of the Liver and Pancreas

+

** The Liver

+

** The Pancreas

+

* V. The Respiratory Tract

+

** Bronchi, Lungs and Air-sacs

+

** The Laryngotracheal Groove

−

V. The Respiratory Tract 325

+

[[Book - The development of the chick (1919) 11|Chapter XI. The Body-Cavities, Mesenteries And Septum Transversum]]

+

* I. The Separation of the Pericardial and Pleuroperi TONEAL Cavities

+

** Septum Transversum

+

** Closure of the Dorsal Opening of the Pericardium

+

** Estahlishment of Independent Pericardial Walls

+

** Derivatives of the Septum Transversum

+

* II. Separation of Pleural and Peritoneal Cavities; Or igin OF THE Septum Pleuro-peritoneale

+

* III. The Mesenteries

+

** The Dorsal Mesentery

+

** The Origin of the Omentum

+

** Origin of the Spleen

−

Bronchi, Lungs and Air-sacs 325

+

[[Book - The development of the chick (1919) 12|Chapter XII. The Later Development Of The Vascular System]]

+

* I. The Heart

+

** The Development of the External Form of the Heart

+

** Division of the Cavities of the Heart

+

** Fate of the Bulbus

+

** The Sinus Ve?iosus

+

* II. The Arterial System

+

** The Aortic Arches

+

** The Carotid Arch

+

** The Subclavian Artery

+

** The Aortic System

+

* III. The Venous System

+

** The Anterior Vence Cavce

+

** The Omphalomesenteric Veins

+

** The Umbilical Veins

+

** The System of the Inferior Vena Cava

+

* IV. The Embryonic Circulation

−

The Laryngotracheal Groove 331

+

[[Book - The development of the chick (1919) 13|Chapter XIII. The Urinogenital System]]

+

* I. The Later History of the Mesonephros

+

* II. The Development of the Metanephros or Permanent

+

** Kidney

+

** The Metanephric Diverticulum

+

** The Nephrogenous Tissue of the Metanephros

+

* III. The Organs of Reproduction

+

** Development of Ovary and Testis

+

** Development of the Genital Duct

+

* IV. The Suprarenal Capsules

+

** Origin of the Cortical Cords

+

** Origin of the Medullary Cords

−

CHAPTER XI. THE BODY-CAVITIES, MESENTERIES AND

+

[[Book - The development of the chick (1919) 13|Chapter XIV. The Skeleton]]

+

* I. General

+

* II. The Vertebral Column

+

** The Sclerotomes and Vertebral Segmentation

+

** Membranous Stage of the Vertebrce

+

** Chondrification

+

** Atlas and Axis (Epistropheus)

+

** Formation of Vertebral Articulations

+

** Ossification

+

* III. Development of the Ribs and Sternal Apparatus

+

* IV. Development of the Skull

+

** Development of the Cartilaginous or Primordial Cranium

+

** Ossification of the Skull

+

* V. Appendicular Skeleton

+

** The Fore-limb

+

** The Skeleton of the Hind-limb

−

SEPTUM TRANSVERSUM 333

+

[[Book - The development of the chick (1919) Appendix|Appendix]]

+

* General Literature

+

* Literature — Chapter I

+

* Literature — Chapter II

+

* Literature — Chapter III

+

* Literature — Chapters IV and V

+

* Literature — Chapter VII

+

* Literature — Chapter VIII

+

* Literature — Chapter IX

+

* Literature — Chapter X

+

* Literature — Chapter XI

+

* Literature — Chapter XII

+

* Literature — Chapter XIII

+

* Literature — Chapter XIV

−

I. The Separation of the Pericardial and Pleuroperi TONEAL Cavities 333

+

Index

−

Septum Transversum 334

+

==Introduction==

−

Closure of the Dorsal Opening of the Pericardium . . . 337

+

===I. The Cell Theory===

−

Estahlishment of Independent Pericardial Walls . . . 338

+

The fundamental basis of the general conceptions of embryology, as of other biological disciplines, is the cell theor3^ The organism is composed of innumerable vital units, the cells, each of which has its independent life. The life of the organism as a whole is a product of the combined activity of all the cells. New cells arise always by subdivision of pre-existing cells, and new generations of the organism from liberated cells of the parental body. The protozoa, however, have the grade of organization of single cells, and the daughter-cells arising by fission constitute at the same time new generations. In some metazoa new generations may arise asexually by a process of budding, as in Hydra, or of fission, as in some Turbellaria; such cases constitute exceptions to the rule that new generations arise from liberated cells of the parental body, but the rule holds without exception for all cases of sexual reproduction.

−

Derivatives of the Septum Transversum 339

+

The body consists of various functional parts or organs; each of these again consists of various tissues, and the tissues are composed of specific kinds of cells. The reproductive organs, or gonads, are characterized by the production of germ-cells, ova in the female gonad or ovary, and spermatozoa in the male gonad or testis. However large the ovum may be, and in the hen it is the part of the egg known as the yolk, it is, nevertheless, a single cell at the time that it leaves the ovary in all animals. Similarly the spermatozoon is a single cell. An ovum and spermatozoon unite, in the manner to be described later, and constitute a single cell by fusion, the fertilized ovum or oosperm. This cell divides and forms two; each of the daughter-cells divides, making four, and the number of cells steadily increases by successive divisions of all daughter-cells, so that a large number of cells is rapidly produced. Organs are formed by successive and orderly differentiation among groups of these cells. Among these organs are the gonads, consisting of cells which trace a continuous lineage by cell-division back to the fertilized ovum, and which are capable of developing into ova or spermatozoa according to the sex of the individual.

The lives of successive generations are thus continuous because the series of germ-cells from which they arise shows no break in continuity. All other kinds of cells composing the body finally die. In view of this contrast the non-germinal cells of the body are known collectively as somatic cells. In some way the germcells of a species maintain very constant properties from generation to generation in spite of their enormous multiplication, and this furnishes the basis for hereditary resemblance.

−

III. The Mesenteries 342

+

The establishment of the fact that in all animals the ovum is a single cell, and that the cells of all tissues of the body are derived from it by a continuous process of cell-division, completes the outline of the cycle of the generations, and furnishes the basis for a complete theory of development. The full significance of this principle can only be appreciated by learning the condition of embryology before the establishment of the cell-theory in the eighteenth century. The history of our knowledge of the development of mammals is particularly instructive in this respect: some knowledge had been gained of the anatomy of the embryos, mostly relatively advanced, of a few^ mammals; but the origin of the embryo was entirely unknown; the ovum itself had not been discovered; the process of fertilization was not understood. In the knowledge of the cycle of generations there was a great gap, and the embryo was as much a mystery as if it had arisen by a direct act of creation. To be sure Harvey in 1651 had propounded the theorem, omne vivum ex ovo, but no one had ever seen the egg of a mammal, and there was no clear idea in the case of other forms what the egg signified.

−

The Dorsal Mesentery 342

+

In 1672, de Graaf (who died in 1673 at the age of 32) published a work, "de mulierum organis generationis inservientibus," in which he attempted to show that the vesicles seen on the surface of the ovaries contained the female reproductive material in bladder-like form. But he could not reconcile this view of the Graafian follicle with the fact that the earliest embryos discovered by him were smaller than the follicles. For this reason his views were opposed by Leeuwenhoek and Valisnieri; and the later researches of Haller and his pupil Kuhlemann seemed to establish a view which l^anished all possibility of a rational explanation of development, viz., that, in the highest group of animals (the mammalia) the embryo arose after fertilization out of formless fluids.

−

The Origin of the Omentum 343

+

In 1827 V. Baer discovered the mammalian ovum within the Graafian follicle. But no correct interpretation of this discovery w^as possible until the establishment of the cell-theory by Theodore Schwann in 1839; Schwann concluded as the result of his investigations that there was one general principle for the formation of all organisms, namely, the formation of cells; that the cause of nutrition and growth resides not in the organism as a whole, but in the separate elementary parts, the cells." He recognized the ovum as a single cell and the germinal vesicle as its nucleus. But on account of his erroneous conception of the origin of cells as a kind of crystallization in a primordial substance, the cytoblastema, he was unable to form the conception of continuity of generations which is an essential part of the modern cell-theory.

−

Origin of the Spleen 345

+

Schwann's theory as regards the ovum was not at once accepted. Indeed, for a period of about twenty years some of the best investigators, notably Bischoff, opposed the view that the ovum is a single cell, and the so-called germinal vesicle its nucleus. It was not, indeed, until 1861 that Gegenbaur decisively demonstrated that the bird's ovimi is a single cell. Even after that it was maintained for a long time by His and his followers that all the cells were not derived from the ovum directly, but that certain tissues, notably the blood and connective tissues, were to be traced to maternal leucocytes that had migrated into the ovum while it was yet in the follicle. This view was decisively disproved in the course of time.

−

CHAPTER XII. THE LATER DEVELOPMENT OF THE VASCULAR SYSTEM 348

+

===II. The Recapitulation Theory===

−

I. The Heart 348

+

Haeckel's formula, that the development of the indi\ddual repeats briefly the evolution of the species, or that ontogeny is a brief recapitulation of phylogeny, has been widely accepted by embryologists. It is based on a comparison between the embryonic development of the individual and the comparative anatomy of the phylum. The embryonic conditions of any set of organs of a higher species of a phylum resemble, in many essential particulars, conditions that are adult in lower species of the same phylum; and, moreover, the order of embryonic development of organs corresponds in general to the taxonomic order of organization of the same organs. As the taxonomic order is the order of evolution, Haeckel's generalization, which he called the fundamental law of biogenesis, w^ould appear to follow^ of necessity.

−

The Development of the External Form of the Heart . . 348

+

But it never happens that the embryo of any definite species resembles in its entirety the adult of a lower species, nor even the embryo of a lower species; its organization is specific at all stages from the ovum on, so that it is possible without any difficulty to recognize the order of animals to which a given embryo belongs, and more careful examination will usually enable one to assign its zoological position very closely.

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Division of the Cavities of the Heart 350

+

If phylogeny be understood to be the succession of adult forms in the line of evolution, it cannot be said in any real sense that ontogeny is a brief recapitulation of phylogeny, for the embryo of a higher form is never like the adult of a lower form, though the anatomy of embryonic organs of higher species resembles in many particulars the anatomy of the homologous organs of the adult of the lower species. However, if w^e conceive that the whole life history is necessary for the definition of a species, we obtain a different basis for the recapitulation theory. The comparable units are then entire ontogenies, and these resemble one another in proportion to the nearness of relationship, just as the definitive structures do. The ontogeny is inherited no less than the adult characteristics, and is subject to precisely the same laws of modification and variation. Thus in nearly related species the ontogenies are very similar; in more distantly related species there is less resemblance, and in species from different classes the ontogenies are widely divergent in many respects.

−

Fate of the Bulbus .357

+

From this it follows that inheritance of the life-history or ontogeny is the fundamental basis of the recapitulation theory. In the course of evolution terminal or late stages of the life history are modified more rapidly in a visible morphological sense, and earlier stages are more conservative in the same sense. Hence ancestral resemblances adhere incomparably longer to the embryo than to the adult. Ontogenies receive something from every stage of evolution, but they retain most of the previous ontogenetic forms, especially of the early stages, in each succeeding evolutionary stage; hence the appearance of recapitulation of the ancestral history.

−

The Sinus Ve?iosus 357

+

Some of these considerations may be represented graphically as follows: let us take a species D that has an ontogeny A, B, C, D, and suppose that this species evolves successively into species E, F, G, H, etc. When evolution has progressed a step, to E, the characters of the species established develop directh' from the ovum, and are therefore, in some way, involved in the composition of the latter. All of the stages of the ontogeny leading up to E are modified, and we can indicate this in the ontogeny

−

II. The Arterial System 358

+

1. A B C D of E as in line 2; similarly, when evolu 2. A^ B^ C^ D^ E tion has progressed to species F, seeing

−

The Aortic Arches 358

+

3. A^ B2 C^ D2 E^ F that the characters of F now develop

−

The Carotid Arch 361

+

4. A^ B^ C^ D^ E2 F^ G directly from the ovum, all the onto 5. A^ B^ C^ D^ E^ F^ G^ H genetic stages leading up to F are modified, line 3. And so on for each successive advance in evolution, lines 4 and 5. It will also be noticed that the terminal stage D of species 1, becomes a successively earlier ontogenetic stage of species 2, 3, 4, 5, etc., and moreover it does not recur in its pure form, but in the form D^ in species 2, D^ in species 3, etc. Now if the last five stages of the ontogeny of species 5 be examined, viz.^ D^ E^, F^, G^ H, it will be seen that they repeat the phylogeny of the adult stages D, E, F, G, H, but in a modified form.

−

The Subclavian Artery 362

+

This is in fact what the diagram shows; but it is an essential defect of the diagram that it is incapable of showing the character of the modifications of the ancestral conditions. Not only is each stage of the ancestral ontogenies modified with each phylogenetic advance, but the elements of organization of the ancestral stages are also dispersed so that no ancestral stage hangs together as a unit. The embryonic stages show as much proportional modification in the course of evolution as the adult, but this is not so obvious owing to the simpler and more generalized character of the embryonic stages.

−

The Aortic System 362

+

The recapitulation theory as outlined above is obviousl}^ a corollary of the theory of organic descent; it was in fact developed in essentially its present form, soon after the publication of the Origin of Species," by Fritz Miiller and Ernst Haeckel. But the data on which it was based were known to the earlier embryologists; and Meckel, for instance, insisted very strongly on the resemblance between the ontogenetic and the taxonomic series (1821). V. Baer opposed Meckel's view that higher organisms pass through the definitive stages of the lower organisms, and formulated his conclusions on the subject in 1828 in the following theses :

−

III. The Venous System ..... c .... . 363

+

# "The more general features of a large division of animals arise in the embryo earlier than the more special features."

+

# "From the most general features of structure arise those that are less general, and so on until the most specific features arise."

+

# "The embryo of any definite species tends away from the specific forms of other species instead of passing through them."

+

# "Fundamentally, therefore, the embryo of any higher species is never like a lower species, but only like its embryo."

−

The Anterior Vence Cavce 363

−

The Omphalomesenteric Veins 364

+

Some embryologists profess to prefer the laws of v. Baer to the recapitulation theory as a formulation of the actual facts. But it is obvious that the only possible explanation of the facts is found in the theory of descent, and that therefore they must be formulated in terms of this theory. The method of formulation will depend on the conception of the nature of the factors of organic evolution. Haeckel stated his theory in Lamarckian terms, which renders it inacceptable in many places to those who cannot accept the Lamarckian point of view. But as the basis of any theory of descent is heredity, and it must be recognized that ontogenies are inherited, the resemblance between the individual history and the phylogenetic history necessarily follows. If one holds, as does the present writer, that phylogenetic variations are germinal in their character, then one must admit that every phase of development of every part has two aspects, viz.: the modern, specific, or coenogenetic, and the ancestral or palingenetic aspect. The latter aspect may be more or less completely obscured in the course of evolution, but it can never entirely vanish because it is the original germ of the specific form acquired. It is not correct from this point of view to classify some features of development as coenogenetic and others as palingenetic, though it is obvious that some characters may exhibit the ancestral conditions in more apparent and others in less apparent form.

−

The Umbilical Veins 367

+

===III. The Physiology of Development===

+

To explain how a germ possessed the potency of forming an adult, the prefor7nationists of the eighteenth century assumed that it contained a miniature adult, and that the process of development consisted essentially in enlargement and completion in detail of that which was already preformed. They solved the problem of development, therefore, by denying its existence: In the begininng the Creator had not only made all species of animals and plants in essentially their present forms, but had at the same time created the germs of all the generations that were ever to come into existence. The ovum of any species, therefore, contained encapsuled the germ of the next generation; this, likewise encapsuled, the germ of the generation next following, and so on to the predetermined end of the species. This was known as the doctrine of evolution or preformation. In opposition to this conception, those of the same period who believed in epigenesis maintained the apparent simplicity of the germ to be real, and development to be actual. But, as there was no conception of the continuity of generations, the adherents of this point of view had to assume the spontaneous generation of the embryo.

−

The System of the Inferior Vena Cava 368

+

A great advance over the preformation theory of development was made in the modern theory of determinants. This conception, which forms the basis of Darwin's theory of pangenesis as well as of Weismann's germ-plasm theory of development, is, essentially, that all the diverse components of the organism are represented in the germ by distinct entities (pangens of Darwin, determinants of Weismann) which are germs of the parts that they represent, and which are so distributed in the process of development that they produce all the parts of the embryo in their proper sequence and relations. This is not the place to enter into the numerous and diverse variations of the determinant hypothesis. It was an advance over the preformation theory of development in so far as it was reconcilable with the cell and protoplasm theories of organization, but it has a real relationship to the preformation theory inasmuch as it denies the simplicity of the germ and avoids any real explanation of the modus operandi of development.

−

IV. The Embryonic Circulation 372

+

Development is as truly a physiological process as secretion, and as such is to be studied by similar methods, mainly experimental. The limits of pure observation without experiment are soon reached in the analysis of such a complex subject as the physiology of development; experiment then becomes necessary to push the analysis of the subject farther^ and to furnish the true interpretation of the observations. In some cases experiments have confirmed the physiological deductions of pure observation, and in many cases have decided between conflicting views. Not all embryological experiments, however, are essays in the direction of a physiologv of development; some are directed to the solution of morphological problems, as, for instance, the origin of the sheath cells of nerves, or the order of origin of somites, or the relation of the primitive streak to the embr3'o. Experimental embryology is, therefore, not synonymous with physiology of development.

−

CHAPTER XIII. THE URINOGENITAL SYSTEM ... 378

+

Physiology of development must proceed from an investigation of the composition and properties of the germ-cells. It must investigate the role of cell-division in development, the factors that determine the location, origin, and properties of the primordia of organs, the laws that determine unequal growth, the conditions that determine the direction of differentiation, the influence of extraorganic conditions on the formation of the embryo, and the effects of the intraorganic environment, i.e., of component parts of the embryo on other parts (correlative differentiation). Each of these divisions of the subject includes numerous problems, which have attracted many investigators, so that the materials for a consistent exposition of the physiology of embryonic development are being rapidly accumulated. This direction of investigation is, however, one of the youngest of the biological disciplines. It will be seen how far it is removed from attempts to explain embryonic development by a single principle.

−

I. The Later History of the Mesonephros 378

+

===IV. Embryonic Primordia and the Law of Genetic Restriction===

−

II. The Development of the Metanephros or Permanent

+

In the course of development the most general features of organization arise first, and those that are successively less general in the order of their specialization. For every structure, therefore, there is a period of emergence from something more general. The earliest discernible germ of any part or organ may be called its primordium. In this sense the ovum is the primordium of the individual, the ectoderm the primordium of all ectodermal structures, the medullary plate the primordium of the central and part of the peripheral nervous system, the first thickening of the ectoderm over the optic cup the primordium of the lens, etc. Primordia are, therefore, of all grades, and each arises from a primordium of a higher grade of generality.

−

Kidney 38-1:

+

The emergence of a primordium involves a limitation in two directions: (1) it is itself limited in a positive fashion by being restricted to a definite line of differentiation more special than the primordium from which it sprang, and (2) the latter is limited in a negative way by losing the capacity for producing another primordium of exactly the same sort. The advance of differentiation sets a limit in all cases, in the manners indicated, to subsequent differentiation, a principle that has been designated by Minot the law of genetic restriction.

−

The Metanephric Diverticulum 384

+

This law has not been sufficiently investigated in an experimental fashion to demonstrate its universal validity, but enough is known to establish its general applicability. A very important property of primordia in many animals is their capacity for subdivision, each part retaining the potencies of the whole. Thus, for instance, in some animals two or several embrvos mav be produced from parts of one ovum. Similarly two or more limbs may be produced in some forms by subdividing a limbbud, etc.

−

The Nephrogenous Tissue of the Metanephros . . . 387

+

===V. General Character of Germ-cells===

−

III. The Organs of Reproduction • 390

+

As already remarked the ovum and spermatozoon have the character of single cells in all animals. They are, however, specialized for the performance of their respective functions. The ovum is relatively large, inert, and usually rounded in form. Its size is due to the presence of a sufficient quantity of protoplasm to serve as the primordium of an embryo, and of a greater or less amount of yolk for its nutrition. The spermatozoon, on the other hand, is relatively minute and capable of locomotion. It contains no food substances, and only sufficient protoplasm to serve as transmitter of paternal qualities and for organs of locomotion.

−

Development of Ovary and Testis 391

−

Development of the Genital Ducts 401

+

The Spermatozoon. The spermatozoon (Fig. 1) is an elongated flagellated cell in which three main divisions are distinguished, viz., head (caput), neck (coUum) and tail (cauda). The head contains the nucleus, and the neck the centrosomes of the sperm mother-cell or spermatid. The tip of the head is often transformed into a perforatorium. Three parts may be recognized in the tail, viz., the connecting piece (pars conjunctionis) next to the neck, frequently called the middle piece, the main piece (pars principalis) and the end-piece or terminal filament (pars terminahs). The entire tail is traversed by an axial filament; in the region of the connecting and main pieces the axial filament is surrounded by a protoplasmic sheath (involucrum) which may be variously modified in different animals. The end-piece is made up of the axial filament alone.

−

IV. The Suprarenal Capsules 403

−

Origin of the Cortical Cords 405

+

The Ovum. The ova of different phyla and classes of animals vary greatly in size, in organization, and in the nature of their envelopes. In considering these variations we shall limit ourselves to the vertebrates. Within the ovary the ovum receives two envelopes, viz., a primary envelope, the so-called vitelline membrane, which is supposed to be secreted by the ovum itself, and a secondary or follicular membrane, which is secreted by the follicular cells. (See Chap. I). Theoretically the distinction between vitelline membrane and follicular membrane (primary and secondary egg-membranes) is perfectly clear; but practically it is impossible in most cases to make such a distinction. Therefore the membrane that surrounds the ovarian ovum will be termed the vitelline membrane or zona radiata without reference to its theoretical mode of origin.

−

Origin of the Medullary Cords 406

−

CHAPTER XIV. THE SKELETON 407

+

The ovum escapes from the ovary (ovulaeon from the vas tion) by rupture of the wall of the follicle, and, deferens, (After -Ballowitz.) ,, 1 1 • u -x -x X 4-1 most vertebrates, is taken up by the oviduct through which it passes on its way to the exterior. Within the oviduct it may become surrounded by tertiary membranes secreted by the wall of the oviduct itself. Tertiary membranes are lacking in some vertebrates, in others they are of great importance. Thus in birds the albumen, the shellmembrane and the shell itself are tertiary membranes.

−

I. General 407

−

II. The Vertebral Column 411

+

The principal differences to be emphasized in the ova of vertebrates are, however, in the amount and arrangement of the yolk contained within the ovum proper. All ova contain more or less yolk. In the case of mammals (excepting the monotremata: Ornithorhynchus, Echidna, etc., which have large ova) the yolk is scanty in amount, and quite uniformly distributed in the form of fine granules; the ovum is, therefore, relatively very small (mouse, 0.059 mm.; man, 0.17 mm.). Such ova are often termed alecithal, which means literally without yolk. In the literal sense, however, no ova are entirely alecithal, so that it will be better to use the term of Waldeyer, isolecithal. In the amphibia the yolk is much greater in amount and it is centered towards one pole of the ovum; the germinal vesicle (nucleus of the egg-cell), which occupies the center of the protoplasm of the ovum, is therefore displaced towards the opposite pole of the ovum. Such ova are termed telolecithal. In the ova of Selachia, reptiles and birds, the yolk is very much greater in amount and in consequence the protoplasm containing the germinal vesicle appears as a small disc, the germinal disc, on the surface of the huge yolk-mass.

−

The Sclerotomes and Vertebral Segmentation .... 412

−

Membranous Stage of the Vertebrce 414

−

Chondrification 418

+

'''Fig. 1.''' — Spermatozoon of the {{pig}}

−

Atlas and Axis (Epistropheus) 420

−

Formation of Vertebral Articulations 421

−

Ossification 421

+

But no matter how large the ovum may become by deposition of yolk, its unicellular character is not altered. The deposition of yolk is simply a provision for the nutrition of the embryo. In the mammals the nutrition of the embryo is provided for by the placenta; therefore yolk may be dispensed with. In the absence of such provision the amount of yolk is a measure of the length of the embryonic period of development. In the amphibia, for instance, this is relatively brief, for the yolk is soon used up, and the larva must then depend on its own activities for its nutrition. Therefore the development involves a metamorphosis: the embryo is born in a very unfinished condition, as a larva (the tadpole in the case of amphibia), which must undergo an extensive metamorphosis to reach the adult condition. In the reptiles and birds, however, the amount of yolk is sufficient to carry the development through to a juvenile condition, before an extraneous food-supply is necessary. The metamorphosis, therefore, which takes place in free life in amphibia, goes on within the egg in reptiles and birds. The first form of development is known as larval, the second as foetal.

−

III. Development of the Ribs and Sternal Apparatus. . 424

−

IV. Development of the Skull 427

+

The amount and arrangement of yolk also influences very profoundly the form of the early stages of development. Ova are classified in this respect as holoblastic and meroblastic. Holoblastic ova are those in which the process of cell division (cleavage or segmentation of the ovum), with which development begins, involves the entire ovum. This occurs where the amount of the yolk is relatively small and where it is completely interpenetrated by sufficient protoplasm to carry the planes of division through the inert volk. But where the amount of yolk becomes very large, or where it is not interpenetrated sufficiently by the protoplasm, the division planes are confined to the protoplasmic portion of the ovum, and the yolk remains undivided. Such ova are known as meroblastic. In these ova the cellular part of the ovum forms a blastodisc (germinal disc) on the surface of the yolk. The ova of Amphioxus, Petromyzontidse, Ganoidea. Dipnoi, Amphibia, Marsupialia, and Placentalia are holoblastic; those of Myxinoidea, Teleostei, Selachia, Reptilia, Aves, and Monotremata are meroblastic.

−

Development of the Cartilaginous or Primordial Cranium. 428

−

Ossification of the Skull 431

+

It is obvious that transitional conditions between holoblastic and meroblastic ova may occur; such are in fact found among the ganoids. In Lepidosteus, for instance, the quantity of protoplasm in the lower hemisphere is so slight that the division planes form with extreme slowness. On the other hand, it should be emphasized that the distinction between holoblastic and meroblastic ova is not so much due to amount of yolk as to the definiteness of its separation from the protoplasm. Thus the ova of some teleosts, particularly of the viviparous forms described by Eigenmann, are many times smaller than the ova of Necturus or Cryptobranchus among amphibia. Yet the teleost ovum is meroblastic, because the protoplasm does not penetrate sufficiently into the yolk, and the amphibian ovum is holoblastic.

−

V. Appendicular Skeleton 434

−

The Fore-limb 434

+

Comparison of the Germ-cells. Although it is not within the province of this book to enter fully into a cUscussion of this question, yet it should be pointed out that, in spite of the extreme differences in the structure of the germ-cells, they are exactly equivalent in hereditary potency, as is proved by the similar nature of reciprocal crosses. Their resemblances are in fact fundamental and their differences must be regarded as adaptations to secure their union. The comparative history of the germ-cells, that is a comparison of ovogenesis and spermatogenesis, brings out their fundamental similarity as germ-cells. In both the ovogenesis and spermatogenesis three periods are clearly distinguishable, viz. : a period of multiplication, a period of growth, and a period of maturation. In the period of multiplication the primordial germ-cells, known as ovogonia and spermatogonia are very similar in their morphological characters; both kinds are small, yolkless cells containing the typical or somatic number of chromosomes; they multiply rapidly by karyokinetic division.

−

The Skeleton of the Hind-limb 438

−

APPENDIX

+

At the end of this period multiplication ceases and the germcells increase in size (period of growth). They are now known as ovocytes and spermatocytes of the first generation. The growth of the ovocyte is much greater than that of the spermatocyte; deposition of yolk occurs in the ovocyte during this period, whereas in the spermatocyte no yolk is ever deposited, though mitochondria may simulate it in appearance. Another characteristic feature of the period of growth is the reduction of the number of chromosomes to one half of the typical number, w^hich takes place, according to the current conception, by union of the chromosomes in pairs (synapsis) forming one half of the somatic number of chromosomes, which are, however, bivalent and are known as tetrads.

−

General Literature ^ •> .... 443

−

Literature — Chapter I 443

+

At the end of the period of growth the ovocyte of the first generation is usually many times larger than the spermatocyte, owing mainly to the amount of yolk formed. But the tw^o kinds of cells are precisely alike in nuclear constitution. Then comes the period of maturation, which is the same in both kinds of cells with reference to the nuclear phenomena, but very different as regards the behavior of the cell-body. The maturation consists of two rapidly succeeding karyokinetic divisions: in the case of the spermatocyte the first division results in the formation of two similar cells, the spermatocytes of the second order, and the second maturation division divides each of these equally, forming two similar spermatids, so that four equal and similar spermatids arise from each spermatocyte of the first order. Each spermatid then differentiates into a single spermatozoon. In the case of the ovocyte of the first order, the first maturation division is exceedingly unequal; the smaller cell is known as the first polar bodv, but both cells are ovocvtes of the second order. The second maturation division usually involves only the large secondary ovocyte; it is as unequal as the first division and results in the formation of a second polar body. The division of the first polar body, where it occurs, is equal. Thus the net result of the maturation division of the ovum is the production of three cells (four if the first polar body divides), viz., the two (or three) polar bodies and the ovum. The size of the polar globules is usually so small that their elimination makes no appreciable difference in the size of the ovum proper, but they have, nevertheless, the same nuclear constitution as the ovum.

−

Literature — Chapter II 444

−

Literature — Chapter III 44o

+

The mature ovum (ootid) and the polar bodies are the precise equivalent of the four spermatids, but whereas each of the latter becomes a functional spermatozoon, only the ovum on the female side is functional; the polar bodies lack the necessary protoplasm and yolk for development, and they therefore die. The polar bodies must be regarded as abortive ova; and a teleological explanation of the form of maturation of the ovum is afforded by the consideration that equal maturation divisions would reduce the amount of protoplasm and yolk in the products below the minimum desirable for perfect development.

−

Literature — Chapters IV and V 44o

−

Literature — Chapter VII 447

+

Although the maturation divisions of the ovum and spermatozoon are so dissimilar externally, yet the nuclear phenomena are exactly alike. The net result of the maturation divisions is to produce definitive germ-cells containing one half of the somatic number of chromosomes owing to the reduction by pairing (synapsis) that occurs in both at the beginning of the period of growth. The somatic number is again restored when the sperm-nucleus and the egg-nucleus unite in fertilization. Questions of fundamental importance for the problems of heredity arise in connection with the phenomena of maturation and fertilization, but their consideration lies without the scope of the present book.

−

Literature — Chapter VIII 449

+

===VI. Polarity and Organization of the Ovum===

−

Literature — Chapter IX 450

+

Although the ovum is morphologically a single cell, yet, as the primordium of an individual, it has certain specific properties that predelineate or foreshadow the main structural features of the embryo. Polarity is the most general of these features: all the axes of the ovum are not similar, though they may be equal; there is one axis around which the development centers; the ends of this axis are known as the animal and the vegetative poles of the ovum, and the hemispheres in which they lie are named correspondingly. In telolecithal ova the yolk is centered in the vegetative hemisphere, the protoplasm in the animal hemisphere; even in ova which are called isolecithal there is a tendency for the yolk to be more abundant in the vegetative hemisphere. The polar globules are formed at the animal pole; hence their name; they often furnish the only clear indication of polarity before cleavage begins.

−

Literature — Chapter X 453

+

With reference to the heteropolar ovic axis a series of meridia may be defined, drawn from pole to pole over the surface; likewise an equator and a series of horizontal zones parallel to the equator. Thus directions on the surface of the ovum may be defined as meridional, equatorial, or oblique.

−

Literature — Chapter XI 4o/

+

Cleavage takes place with reference to the axis of the ovum. Thus in holoblastic vertebrate ova the first and second cleavage planes are meridional, and the third usually equatorial. The mammalian ovum may form an exception to this rule, though little is known, as a matter of fact, about the polarity of the mammalian ovum. The cleavage of meroblastic ova takes place likewise with reference to the polarity (see Chap. II); and the location of the primary germ-layers is determined by the polarity.

−

Literature — Chapter XII 458

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Not only is the ovum heteropolar, but in many bilateral animals, and perhaps in all, it is bilaterally symmetrical before cleavage begins; that is to say, one of the meridional planes defines the longitudinal axis of the future embryo, and the direction of anterior and posterior ends is also predetermined in this meridian, so that halves of the egg corresponding to future right and left sides of the embryo may be distinguished. In the frog's egg the plane of symmetry is marked by a gray crescent that appears above the equator on the side of the egg that corresponds to the hinder end of the embryo. This crescent is bisected by the meridional plane of symmetry. In the hen's egg the plane of symmetry of the embryo appears on the surface of the yolk in a line at right angles to the axis of the shell, and the left side of the embryo is turned towards the broad end, the right side towards the narrow end of the shell. The same plane of symmetry must exist in the ovum prior to cleavage for reasons explained beyond, although there is no morphological differentiation in the ovum proper, i.e., the germinal disc or yolk, that indicates it.

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Literature — Chapter XIII 459

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This predelineation of embryonic axes within the unsegmented ovum has been interpreted physiologically as due to gradients in rate of metabolic processes along the embryonic axes (Child), which determine the locaUzation of the main developmental events.

The writing of the present edition was begun in 1945 at the request of Dr Frank R. Lillie himself with Dr B. H. Willier acting as advisory editor. It was Dr Lillie’s hope that he might live to see the new edition in print but this was not to be. The general outline of previous editions has been preserved. Part 1, which consists of six chapters, is devoted to an account of the early embryology up to and including the 3rd day. The account of the development of the embryo is given on a general basis and in addition a detailed account is given of specially selected stages.

Part 2 of the book consists of nine chapters and is an account of the development of the embryo from the 4th day to hatching; the various systems and external form are described as separate entities. A few chapters, such as the one dealing with the external form of the embryo and the embryonic membranes, and the one describing the body cavities, mesenteries and septum transversum, have remained relatively unchanged. Chapter 4, ‘From laying to the formation of the first somite’, chapter 8; ‘The nervous system’, and chapter 13, ‘The urogenital system’, are more or less completely rewritten. A new chapter, the fifteenth, describing the development of the integument, has been added. The other chapters have been extensively revised.

The new accounts are based on recent literature, but the author has tried to follow Dr Lillie’s example of going to the chick itself to check questionable points. To this end some original work is included in the text, but it is to be regretted that the author has not indicated more clearly which parts of the text result from this original work. The only clear indications consist of an opinion on the processes concerned with the formation of endoderm (p. 101) and two footnotes, one dealing with the coelomic cavity (p. 149) and one with the tail bud (p. 176). A further footnote refers to a communication from Rawles on the patency of the ductus arteriosus in the newly-hatched chick (p. 462).

This book is very well written and its format is attractive. The book reaches a happy compromise which makes it a most readable introduction to embryology while yet remaining an invaluable reference work for the research worker.

There is little to criticize in this work which has evidently been prepared with great care, but future editions might be improved by a rearrangement of the bibliography. The references should be listed at the end of the chapter they concern and not in an appendix of 32 pages at the end of the book. Also the magnification of drawings and photographs of early embryos should be given. Figs. 153 and 155 would be improved by being photographs rather than drawings of sagittal sections through an embryo. In fig. 222 the drawings are too small and too faint.

Apart from these minor faults the present work is a credit to the author and had Dr Lillie lived he would have been proud to have his name associated with it. It will continue to perpetuate Dr Lillie’s influence on the development of embryology.

Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding. (More? Embryology History | Historic Embryology Papers)

The Development of the Chick - An Introduction to Embryology

Preface to First Edition

This book is a plain account of the development of the neverfailing resource of the embryologist, the chick. It has been necessary to fill certain gaps in our knowledge of the development of the chick by descriptions of other birds. But the account does not go beyond the class Aves, and it applies exclusively to the chick except where there is specific statement to the contrary. Projected chapters on the integument, muscular system, physiology of development, teratology, and history of the subject have been omitted, as the book seemed to be already sufficiently long. The account has been written directly from the material in almost every part, and it has involved some special investigations, particularly on the early development undertaken by Doctor Mary Blount and Doctor J. T. Patterson, to whom acknowledgments are due for permission to incorporate their results before full publication by the authors. As the book is meant for the use of beginners in embryology, references to authors are usually omitted except where the account is based directly on the description of a single investigator. A fairly full list of original sources is published as an appendix.

Figures borrowed from other publications are credited in the legends to the figures. The majority of the illustrations are from original preparations of the author: Figures 46, 48, 50, 51, 52, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 71, 72, 73, 74, 75, 99, 105 and 106 were drawn by Mr. K. Hayashi; the remainder of the original drawings were executed by Mr. Kenji Toda. The photographs in Figures 118, 119, 120, 168, 181, 182, 189, 194, 197, and 231 are the work of Mr. Willard C. Green. Some of the figures may be studied with advantage for points not described in the text.

Acknowledgments are also due my colleague, Professor W. L. Tower for much assistance, and to Doctor Rov L. Moodie for special work on the skeleton, and photographs of potash preparations reproduced in Figures 242, 246, 249 and 250.

The best introduction to the problems opened up by the study of embryology is a careful first-hand study of some one species. It is in this sense that the book may serve as an introduction to embryology, if its study is accompanied by careful laboratory work. In some respects it is fuller, and in others less complete, than other books with which it might be compared. On its comparative and experimental sides, embryology is the only key to the solution of some of the most fundamental problems of biology. The fact that comparative and experimental embryology receive bare mention is not due to any lack of appreciation of their interest and importance, but to the conviction that the beginner is not prepared to appreciate these problems at the start; to the belief that our teachers of embryology are competent to remedy omissions; and finally to the circumstance that no one book can, as a matter of fact, cover the entire field, except in the most superficial way.

The development before laying and the first three days of incubation are treated by stages as far as possible, and this matter constitutes Part I of the book. It involves the study of the origin of the primordia of most of the organs. The matter concerning the later development is classified by the organs concerned, which seems to be the only possible way, and this constitutes Part II. The first part is complete in itself, so far as it goes, and no doubt it will be the only part consulted by some students.

The attempt to present a consecutive account of the development of the form on which so many classics in the history of embryology have been based is no slight undertaking. The author can hardly hope that he has avoided omissions and errors, and he will be sincerely grateful to those who call such to his attention.

Review - Lillie’s Development of the Chicken - an Introduction to Embryology 3rd Edn. (1952)

The writing of the present edition was begun in 1945 at the request of Dr Frank R. Lillie himself with Dr B. H. Willier acting as advisory editor. It was Dr Lillie’s hope that he might live to see the new edition in print but this was not to be. The general outline of previous editions has been preserved. Part 1, which consists of six chapters, is devoted to an account of the early embryology up to and including the 3rd day. The account of the development of the embryo is given on a general basis and in addition a detailed account is given of specially selected stages.

Part 2 of the book consists of nine chapters and is an account of the development of the embryo from the 4th day to hatching; the various systems and external form are described as separate entities. A few chapters, such as the one dealing with the external form of the embryo and the embryonic membranes, and the one describing the body cavities, mesenteries and septum transversum, have remained relatively unchanged. Chapter 4, ‘From laying to the formation of the first somite’, chapter 8; ‘The nervous system’, and chapter 13, ‘The urogenital system’, are more or less completely rewritten. A new chapter, the fifteenth, describing the development of the integument, has been added. The other chapters have been extensively revised.

The new accounts are based on recent literature, but the author has tried to follow Dr Lillie’s example of going to the chick itself to check questionable points. To this end some original work is included in the text, but it is to be regretted that the author has not indicated more clearly which parts of the text result from this original work. The only clear indications consist of an opinion on the processes concerned with the formation of endoderm (p. 101) and two footnotes, one dealing with the coelomic cavity (p. 149) and one with the tail bud (p. 176). A further footnote refers to a communication from Rawles on the patency of the ductus arteriosus in the newly-hatched chick (p. 462).

This book is very well written and its format is attractive. The book reaches a happy compromise which makes it a most readable introduction to embryology while yet remaining an invaluable reference work for the research worker.

There is little to criticize in this work which has evidently been prepared with great care, but future editions might be improved by a rearrangement of the bibliography. The references should be listed at the end of the chapter they concern and not in an appendix of 32 pages at the end of the book. Also the magnification of drawings and photographs of early embryos should be given. Figs. 153 and 155 would be improved by being photographs rather than drawings of sagittal sections through an embryo. In fig. 222 the drawings are too small and too faint.

Apart from these minor faults the present work is a credit to the author and had Dr Lillie lived he would have been proud to have his name associated with it. It will continue to perpetuate Dr Lillie’s influence on the development of embryology.

Introduction

I. The Cell Theory

The fundamental basis of the general conceptions of embryology, as of other biological disciplines, is the cell theor3^ The organism is composed of innumerable vital units, the cells, each of which has its independent life. The life of the organism as a whole is a product of the combined activity of all the cells. New cells arise always by subdivision of pre-existing cells, and new generations of the organism from liberated cells of the parental body. The protozoa, however, have the grade of organization of single cells, and the daughter-cells arising by fission constitute at the same time new generations. In some metazoa new generations may arise asexually by a process of budding, as in Hydra, or of fission, as in some Turbellaria; such cases constitute exceptions to the rule that new generations arise from liberated cells of the parental body, but the rule holds without exception for all cases of sexual reproduction.

The body consists of various functional parts or organs; each of these again consists of various tissues, and the tissues are composed of specific kinds of cells. The reproductive organs, or gonads, are characterized by the production of germ-cells, ova in the female gonad or ovary, and spermatozoa in the male gonad or testis. However large the ovum may be, and in the hen it is the part of the egg known as the yolk, it is, nevertheless, a single cell at the time that it leaves the ovary in all animals. Similarly the spermatozoon is a single cell. An ovum and spermatozoon unite, in the manner to be described later, and constitute a single cell by fusion, the fertilized ovum or oosperm. This cell divides and forms two; each of the daughter-cells divides, making four, and the number of cells steadily increases by successive divisions of all daughter-cells, so that a large number of cells is rapidly produced. Organs are formed by successive and orderly differentiation among groups of these cells. Among these organs are the gonads, consisting of cells which trace a continuous lineage by cell-division back to the fertilized ovum, and which are capable of developing into ova or spermatozoa according to the sex of the individual.

The lives of successive generations are thus continuous because the series of germ-cells from which they arise shows no break in continuity. All other kinds of cells composing the body finally die. In view of this contrast the non-germinal cells of the body are known collectively as somatic cells. In some way the germcells of a species maintain very constant properties from generation to generation in spite of their enormous multiplication, and this furnishes the basis for hereditary resemblance.

The establishment of the fact that in all animals the ovum is a single cell, and that the cells of all tissues of the body are derived from it by a continuous process of cell-division, completes the outline of the cycle of the generations, and furnishes the basis for a complete theory of development. The full significance of this principle can only be appreciated by learning the condition of embryology before the establishment of the cell-theory in the eighteenth century. The history of our knowledge of the development of mammals is particularly instructive in this respect: some knowledge had been gained of the anatomy of the embryos, mostly relatively advanced, of a few^ mammals; but the origin of the embryo was entirely unknown; the ovum itself had not been discovered; the process of fertilization was not understood. In the knowledge of the cycle of generations there was a great gap, and the embryo was as much a mystery as if it had arisen by a direct act of creation. To be sure Harvey in 1651 had propounded the theorem, omne vivum ex ovo, but no one had ever seen the egg of a mammal, and there was no clear idea in the case of other forms what the egg signified.

In 1672, de Graaf (who died in 1673 at the age of 32) published a work, "de mulierum organis generationis inservientibus," in which he attempted to show that the vesicles seen on the surface of the ovaries contained the female reproductive material in bladder-like form. But he could not reconcile this view of the Graafian follicle with the fact that the earliest embryos discovered by him were smaller than the follicles. For this reason his views were opposed by Leeuwenhoek and Valisnieri; and the later researches of Haller and his pupil Kuhlemann seemed to establish a view which l^anished all possibility of a rational explanation of development, viz., that, in the highest group of animals (the mammalia) the embryo arose after fertilization out of formless fluids.

In 1827 V. Baer discovered the mammalian ovum within the Graafian follicle. But no correct interpretation of this discovery w^as possible until the establishment of the cell-theory by Theodore Schwann in 1839; Schwann concluded as the result of his investigations that there was one general principle for the formation of all organisms, namely, the formation of cells; that the cause of nutrition and growth resides not in the organism as a whole, but in the separate elementary parts, the cells." He recognized the ovum as a single cell and the germinal vesicle as its nucleus. But on account of his erroneous conception of the origin of cells as a kind of crystallization in a primordial substance, the cytoblastema, he was unable to form the conception of continuity of generations which is an essential part of the modern cell-theory.

Schwann's theory as regards the ovum was not at once accepted. Indeed, for a period of about twenty years some of the best investigators, notably Bischoff, opposed the view that the ovum is a single cell, and the so-called germinal vesicle its nucleus. It was not, indeed, until 1861 that Gegenbaur decisively demonstrated that the bird's ovimi is a single cell. Even after that it was maintained for a long time by His and his followers that all the cells were not derived from the ovum directly, but that certain tissues, notably the blood and connective tissues, were to be traced to maternal leucocytes that had migrated into the ovum while it was yet in the follicle. This view was decisively disproved in the course of time.

II. The Recapitulation Theory

Haeckel's formula, that the development of the indi\ddual repeats briefly the evolution of the species, or that ontogeny is a brief recapitulation of phylogeny, has been widely accepted by embryologists. It is based on a comparison between the embryonic development of the individual and the comparative anatomy of the phylum. The embryonic conditions of any set of organs of a higher species of a phylum resemble, in many essential particulars, conditions that are adult in lower species of the same phylum; and, moreover, the order of embryonic development of organs corresponds in general to the taxonomic order of organization of the same organs. As the taxonomic order is the order of evolution, Haeckel's generalization, which he called the fundamental law of biogenesis, w^ould appear to follow^ of necessity.

But it never happens that the embryo of any definite species resembles in its entirety the adult of a lower species, nor even the embryo of a lower species; its organization is specific at all stages from the ovum on, so that it is possible without any difficulty to recognize the order of animals to which a given embryo belongs, and more careful examination will usually enable one to assign its zoological position very closely.

If phylogeny be understood to be the succession of adult forms in the line of evolution, it cannot be said in any real sense that ontogeny is a brief recapitulation of phylogeny, for the embryo of a higher form is never like the adult of a lower form, though the anatomy of embryonic organs of higher species resembles in many particulars the anatomy of the homologous organs of the adult of the lower species. However, if w^e conceive that the whole life history is necessary for the definition of a species, we obtain a different basis for the recapitulation theory. The comparable units are then entire ontogenies, and these resemble one another in proportion to the nearness of relationship, just as the definitive structures do. The ontogeny is inherited no less than the adult characteristics, and is subject to precisely the same laws of modification and variation. Thus in nearly related species the ontogenies are very similar; in more distantly related species there is less resemblance, and in species from different classes the ontogenies are widely divergent in many respects.

From this it follows that inheritance of the life-history or ontogeny is the fundamental basis of the recapitulation theory. In the course of evolution terminal or late stages of the life history are modified more rapidly in a visible morphological sense, and earlier stages are more conservative in the same sense. Hence ancestral resemblances adhere incomparably longer to the embryo than to the adult. Ontogenies receive something from every stage of evolution, but they retain most of the previous ontogenetic forms, especially of the early stages, in each succeeding evolutionary stage; hence the appearance of recapitulation of the ancestral history.

Some of these considerations may be represented graphically as follows: let us take a species D that has an ontogeny A, B, C, D, and suppose that this species evolves successively into species E, F, G, H, etc. When evolution has progressed a step, to E, the characters of the species established develop directh' from the ovum, and are therefore, in some way, involved in the composition of the latter. All of the stages of the ontogeny leading up to E are modified, and we can indicate this in the ontogeny

1. A B C D of E as in line 2; similarly, when evolu 2. A^ B^ C^ D^ E tion has progressed to species F, seeing

3. A^ B2 C^ D2 E^ F that the characters of F now develop

4. A^ B^ C^ D^ E2 F^ G directly from the ovum, all the onto 5. A^ B^ C^ D^ E^ F^ G^ H genetic stages leading up to F are modified, line 3. And so on for each successive advance in evolution, lines 4 and 5. It will also be noticed that the terminal stage D of species 1, becomes a successively earlier ontogenetic stage of species 2, 3, 4, 5, etc., and moreover it does not recur in its pure form, but in the form D^ in species 2, D^ in species 3, etc. Now if the last five stages of the ontogeny of species 5 be examined, viz.^ D^ E^, F^, G^ H, it will be seen that they repeat the phylogeny of the adult stages D, E, F, G, H, but in a modified form.

This is in fact what the diagram shows; but it is an essential defect of the diagram that it is incapable of showing the character of the modifications of the ancestral conditions. Not only is each stage of the ancestral ontogenies modified with each phylogenetic advance, but the elements of organization of the ancestral stages are also dispersed so that no ancestral stage hangs together as a unit. The embryonic stages show as much proportional modification in the course of evolution as the adult, but this is not so obvious owing to the simpler and more generalized character of the embryonic stages.

The recapitulation theory as outlined above is obviousl}^ a corollary of the theory of organic descent; it was in fact developed in essentially its present form, soon after the publication of the Origin of Species," by Fritz Miiller and Ernst Haeckel. But the data on which it was based were known to the earlier embryologists; and Meckel, for instance, insisted very strongly on the resemblance between the ontogenetic and the taxonomic series (1821). V. Baer opposed Meckel's view that higher organisms pass through the definitive stages of the lower organisms, and formulated his conclusions on the subject in 1828 in the following theses :

"The more general features of a large division of animals arise in the embryo earlier than the more special features."

"From the most general features of structure arise those that are less general, and so on until the most specific features arise."

"The embryo of any definite species tends away from the specific forms of other species instead of passing through them."

"Fundamentally, therefore, the embryo of any higher species is never like a lower species, but only like its embryo."

Some embryologists profess to prefer the laws of v. Baer to the recapitulation theory as a formulation of the actual facts. But it is obvious that the only possible explanation of the facts is found in the theory of descent, and that therefore they must be formulated in terms of this theory. The method of formulation will depend on the conception of the nature of the factors of organic evolution. Haeckel stated his theory in Lamarckian terms, which renders it inacceptable in many places to those who cannot accept the Lamarckian point of view. But as the basis of any theory of descent is heredity, and it must be recognized that ontogenies are inherited, the resemblance between the individual history and the phylogenetic history necessarily follows. If one holds, as does the present writer, that phylogenetic variations are germinal in their character, then one must admit that every phase of development of every part has two aspects, viz.: the modern, specific, or coenogenetic, and the ancestral or palingenetic aspect. The latter aspect may be more or less completely obscured in the course of evolution, but it can never entirely vanish because it is the original germ of the specific form acquired. It is not correct from this point of view to classify some features of development as coenogenetic and others as palingenetic, though it is obvious that some characters may exhibit the ancestral conditions in more apparent and others in less apparent form.

III. The Physiology of Development

To explain how a germ possessed the potency of forming an adult, the prefor7nationists of the eighteenth century assumed that it contained a miniature adult, and that the process of development consisted essentially in enlargement and completion in detail of that which was already preformed. They solved the problem of development, therefore, by denying its existence: In the begininng the Creator had not only made all species of animals and plants in essentially their present forms, but had at the same time created the germs of all the generations that were ever to come into existence. The ovum of any species, therefore, contained encapsuled the germ of the next generation; this, likewise encapsuled, the germ of the generation next following, and so on to the predetermined end of the species. This was known as the doctrine of evolution or preformation. In opposition to this conception, those of the same period who believed in epigenesis maintained the apparent simplicity of the germ to be real, and development to be actual. But, as there was no conception of the continuity of generations, the adherents of this point of view had to assume the spontaneous generation of the embryo.

A great advance over the preformation theory of development was made in the modern theory of determinants. This conception, which forms the basis of Darwin's theory of pangenesis as well as of Weismann's germ-plasm theory of development, is, essentially, that all the diverse components of the organism are represented in the germ by distinct entities (pangens of Darwin, determinants of Weismann) which are germs of the parts that they represent, and which are so distributed in the process of development that they produce all the parts of the embryo in their proper sequence and relations. This is not the place to enter into the numerous and diverse variations of the determinant hypothesis. It was an advance over the preformation theory of development in so far as it was reconcilable with the cell and protoplasm theories of organization, but it has a real relationship to the preformation theory inasmuch as it denies the simplicity of the germ and avoids any real explanation of the modus operandi of development.

Development is as truly a physiological process as secretion, and as such is to be studied by similar methods, mainly experimental. The limits of pure observation without experiment are soon reached in the analysis of such a complex subject as the physiology of development; experiment then becomes necessary to push the analysis of the subject farther^ and to furnish the true interpretation of the observations. In some cases experiments have confirmed the physiological deductions of pure observation, and in many cases have decided between conflicting views. Not all embryological experiments, however, are essays in the direction of a physiologv of development; some are directed to the solution of morphological problems, as, for instance, the origin of the sheath cells of nerves, or the order of origin of somites, or the relation of the primitive streak to the embr3'o. Experimental embryology is, therefore, not synonymous with physiology of development.

Physiology of development must proceed from an investigation of the composition and properties of the germ-cells. It must investigate the role of cell-division in development, the factors that determine the location, origin, and properties of the primordia of organs, the laws that determine unequal growth, the conditions that determine the direction of differentiation, the influence of extraorganic conditions on the formation of the embryo, and the effects of the intraorganic environment, i.e., of component parts of the embryo on other parts (correlative differentiation). Each of these divisions of the subject includes numerous problems, which have attracted many investigators, so that the materials for a consistent exposition of the physiology of embryonic development are being rapidly accumulated. This direction of investigation is, however, one of the youngest of the biological disciplines. It will be seen how far it is removed from attempts to explain embryonic development by a single principle.

IV. Embryonic Primordia and the Law of Genetic Restriction

In the course of development the most general features of organization arise first, and those that are successively less general in the order of their specialization. For every structure, therefore, there is a period of emergence from something more general. The earliest discernible germ of any part or organ may be called its primordium. In this sense the ovum is the primordium of the individual, the ectoderm the primordium of all ectodermal structures, the medullary plate the primordium of the central and part of the peripheral nervous system, the first thickening of the ectoderm over the optic cup the primordium of the lens, etc. Primordia are, therefore, of all grades, and each arises from a primordium of a higher grade of generality.

The emergence of a primordium involves a limitation in two directions: (1) it is itself limited in a positive fashion by being restricted to a definite line of differentiation more special than the primordium from which it sprang, and (2) the latter is limited in a negative way by losing the capacity for producing another primordium of exactly the same sort. The advance of differentiation sets a limit in all cases, in the manners indicated, to subsequent differentiation, a principle that has been designated by Minot the law of genetic restriction.

This law has not been sufficiently investigated in an experimental fashion to demonstrate its universal validity, but enough is known to establish its general applicability. A very important property of primordia in many animals is their capacity for subdivision, each part retaining the potencies of the whole. Thus, for instance, in some animals two or several embrvos mav be produced from parts of one ovum. Similarly two or more limbs may be produced in some forms by subdividing a limbbud, etc.

V. General Character of Germ-cells

As already remarked the ovum and spermatozoon have the character of single cells in all animals. They are, however, specialized for the performance of their respective functions. The ovum is relatively large, inert, and usually rounded in form. Its size is due to the presence of a sufficient quantity of protoplasm to serve as the primordium of an embryo, and of a greater or less amount of yolk for its nutrition. The spermatozoon, on the other hand, is relatively minute and capable of locomotion. It contains no food substances, and only sufficient protoplasm to serve as transmitter of paternal qualities and for organs of locomotion.

The Spermatozoon. The spermatozoon (Fig. 1) is an elongated flagellated cell in which three main divisions are distinguished, viz., head (caput), neck (coUum) and tail (cauda). The head contains the nucleus, and the neck the centrosomes of the sperm mother-cell or spermatid. The tip of the head is often transformed into a perforatorium. Three parts may be recognized in the tail, viz., the connecting piece (pars conjunctionis) next to the neck, frequently called the middle piece, the main piece (pars principalis) and the end-piece or terminal filament (pars terminahs). The entire tail is traversed by an axial filament; in the region of the connecting and main pieces the axial filament is surrounded by a protoplasmic sheath (involucrum) which may be variously modified in different animals. The end-piece is made up of the axial filament alone.

The Ovum. The ova of different phyla and classes of animals vary greatly in size, in organization, and in the nature of their envelopes. In considering these variations we shall limit ourselves to the vertebrates. Within the ovary the ovum receives two envelopes, viz., a primary envelope, the so-called vitelline membrane, which is supposed to be secreted by the ovum itself, and a secondary or follicular membrane, which is secreted by the follicular cells. (See Chap. I). Theoretically the distinction between vitelline membrane and follicular membrane (primary and secondary egg-membranes) is perfectly clear; but practically it is impossible in most cases to make such a distinction. Therefore the membrane that surrounds the ovarian ovum will be termed the vitelline membrane or zona radiata without reference to its theoretical mode of origin.

The ovum escapes from the ovary (ovulaeon from the vas tion) by rupture of the wall of the follicle, and, deferens, (After -Ballowitz.) ,, 1 1 • u -x -x X 4-1 most vertebrates, is taken up by the oviduct through which it passes on its way to the exterior. Within the oviduct it may become surrounded by tertiary membranes secreted by the wall of the oviduct itself. Tertiary membranes are lacking in some vertebrates, in others they are of great importance. Thus in birds the albumen, the shellmembrane and the shell itself are tertiary membranes.

The principal differences to be emphasized in the ova of vertebrates are, however, in the amount and arrangement of the yolk contained within the ovum proper. All ova contain more or less yolk. In the case of mammals (excepting the monotremata: Ornithorhynchus, Echidna, etc., which have large ova) the yolk is scanty in amount, and quite uniformly distributed in the form of fine granules; the ovum is, therefore, relatively very small (mouse, 0.059 mm.; man, 0.17 mm.). Such ova are often termed alecithal, which means literally without yolk. In the literal sense, however, no ova are entirely alecithal, so that it will be better to use the term of Waldeyer, isolecithal. In the amphibia the yolk is much greater in amount and it is centered towards one pole of the ovum; the germinal vesicle (nucleus of the egg-cell), which occupies the center of the protoplasm of the ovum, is therefore displaced towards the opposite pole of the ovum. Such ova are termed telolecithal. In the ova of Selachia, reptiles and birds, the yolk is very much greater in amount and in consequence the protoplasm containing the germinal vesicle appears as a small disc, the germinal disc, on the surface of the huge yolk-mass.

But no matter how large the ovum may become by deposition of yolk, its unicellular character is not altered. The deposition of yolk is simply a provision for the nutrition of the embryo. In the mammals the nutrition of the embryo is provided for by the placenta; therefore yolk may be dispensed with. In the absence of such provision the amount of yolk is a measure of the length of the embryonic period of development. In the amphibia, for instance, this is relatively brief, for the yolk is soon used up, and the larva must then depend on its own activities for its nutrition. Therefore the development involves a metamorphosis: the embryo is born in a very unfinished condition, as a larva (the tadpole in the case of amphibia), which must undergo an extensive metamorphosis to reach the adult condition. In the reptiles and birds, however, the amount of yolk is sufficient to carry the development through to a juvenile condition, before an extraneous food-supply is necessary. The metamorphosis, therefore, which takes place in free life in amphibia, goes on within the egg in reptiles and birds. The first form of development is known as larval, the second as foetal.

The amount and arrangement of yolk also influences very profoundly the form of the early stages of development. Ova are classified in this respect as holoblastic and meroblastic. Holoblastic ova are those in which the process of cell division (cleavage or segmentation of the ovum), with which development begins, involves the entire ovum. This occurs where the amount of the yolk is relatively small and where it is completely interpenetrated by sufficient protoplasm to carry the planes of division through the inert volk. But where the amount of yolk becomes very large, or where it is not interpenetrated sufficiently by the protoplasm, the division planes are confined to the protoplasmic portion of the ovum, and the yolk remains undivided. Such ova are known as meroblastic. In these ova the cellular part of the ovum forms a blastodisc (germinal disc) on the surface of the yolk. The ova of Amphioxus, Petromyzontidse, Ganoidea. Dipnoi, Amphibia, Marsupialia, and Placentalia are holoblastic; those of Myxinoidea, Teleostei, Selachia, Reptilia, Aves, and Monotremata are meroblastic.

It is obvious that transitional conditions between holoblastic and meroblastic ova may occur; such are in fact found among the ganoids. In Lepidosteus, for instance, the quantity of protoplasm in the lower hemisphere is so slight that the division planes form with extreme slowness. On the other hand, it should be emphasized that the distinction between holoblastic and meroblastic ova is not so much due to amount of yolk as to the definiteness of its separation from the protoplasm. Thus the ova of some teleosts, particularly of the viviparous forms described by Eigenmann, are many times smaller than the ova of Necturus or Cryptobranchus among amphibia. Yet the teleost ovum is meroblastic, because the protoplasm does not penetrate sufficiently into the yolk, and the amphibian ovum is holoblastic.

Comparison of the Germ-cells. Although it is not within the province of this book to enter fully into a cUscussion of this question, yet it should be pointed out that, in spite of the extreme differences in the structure of the germ-cells, they are exactly equivalent in hereditary potency, as is proved by the similar nature of reciprocal crosses. Their resemblances are in fact fundamental and their differences must be regarded as adaptations to secure their union. The comparative history of the germ-cells, that is a comparison of ovogenesis and spermatogenesis, brings out their fundamental similarity as germ-cells. In both the ovogenesis and spermatogenesis three periods are clearly distinguishable, viz. : a period of multiplication, a period of growth, and a period of maturation. In the period of multiplication the primordial germ-cells, known as ovogonia and spermatogonia are very similar in their morphological characters; both kinds are small, yolkless cells containing the typical or somatic number of chromosomes; they multiply rapidly by karyokinetic division.

At the end of this period multiplication ceases and the germcells increase in size (period of growth). They are now known as ovocytes and spermatocytes of the first generation. The growth of the ovocyte is much greater than that of the spermatocyte; deposition of yolk occurs in the ovocyte during this period, whereas in the spermatocyte no yolk is ever deposited, though mitochondria may simulate it in appearance. Another characteristic feature of the period of growth is the reduction of the number of chromosomes to one half of the typical number, w^hich takes place, according to the current conception, by union of the chromosomes in pairs (synapsis) forming one half of the somatic number of chromosomes, which are, however, bivalent and are known as tetrads.

At the end of the period of growth the ovocyte of the first generation is usually many times larger than the spermatocyte, owing mainly to the amount of yolk formed. But the tw^o kinds of cells are precisely alike in nuclear constitution. Then comes the period of maturation, which is the same in both kinds of cells with reference to the nuclear phenomena, but very different as regards the behavior of the cell-body. The maturation consists of two rapidly succeeding karyokinetic divisions: in the case of the spermatocyte the first division results in the formation of two similar cells, the spermatocytes of the second order, and the second maturation division divides each of these equally, forming two similar spermatids, so that four equal and similar spermatids arise from each spermatocyte of the first order. Each spermatid then differentiates into a single spermatozoon. In the case of the ovocyte of the first order, the first maturation division is exceedingly unequal; the smaller cell is known as the first polar bodv, but both cells are ovocvtes of the second order. The second maturation division usually involves only the large secondary ovocyte; it is as unequal as the first division and results in the formation of a second polar body. The division of the first polar body, where it occurs, is equal. Thus the net result of the maturation division of the ovum is the production of three cells (four if the first polar body divides), viz., the two (or three) polar bodies and the ovum. The size of the polar globules is usually so small that their elimination makes no appreciable difference in the size of the ovum proper, but they have, nevertheless, the same nuclear constitution as the ovum.

The mature ovum (ootid) and the polar bodies are the precise equivalent of the four spermatids, but whereas each of the latter becomes a functional spermatozoon, only the ovum on the female side is functional; the polar bodies lack the necessary protoplasm and yolk for development, and they therefore die. The polar bodies must be regarded as abortive ova; and a teleological explanation of the form of maturation of the ovum is afforded by the consideration that equal maturation divisions would reduce the amount of protoplasm and yolk in the products below the minimum desirable for perfect development.

Although the maturation divisions of the ovum and spermatozoon are so dissimilar externally, yet the nuclear phenomena are exactly alike. The net result of the maturation divisions is to produce definitive germ-cells containing one half of the somatic number of chromosomes owing to the reduction by pairing (synapsis) that occurs in both at the beginning of the period of growth. The somatic number is again restored when the sperm-nucleus and the egg-nucleus unite in fertilization. Questions of fundamental importance for the problems of heredity arise in connection with the phenomena of maturation and fertilization, but their consideration lies without the scope of the present book.

VI. Polarity and Organization of the Ovum

Although the ovum is morphologically a single cell, yet, as the primordium of an individual, it has certain specific properties that predelineate or foreshadow the main structural features of the embryo. Polarity is the most general of these features: all the axes of the ovum are not similar, though they may be equal; there is one axis around which the development centers; the ends of this axis are known as the animal and the vegetative poles of the ovum, and the hemispheres in which they lie are named correspondingly. In telolecithal ova the yolk is centered in the vegetative hemisphere, the protoplasm in the animal hemisphere; even in ova which are called isolecithal there is a tendency for the yolk to be more abundant in the vegetative hemisphere. The polar globules are formed at the animal pole; hence their name; they often furnish the only clear indication of polarity before cleavage begins.

With reference to the heteropolar ovic axis a series of meridia may be defined, drawn from pole to pole over the surface; likewise an equator and a series of horizontal zones parallel to the equator. Thus directions on the surface of the ovum may be defined as meridional, equatorial, or oblique.

Cleavage takes place with reference to the axis of the ovum. Thus in holoblastic vertebrate ova the first and second cleavage planes are meridional, and the third usually equatorial. The mammalian ovum may form an exception to this rule, though little is known, as a matter of fact, about the polarity of the mammalian ovum. The cleavage of meroblastic ova takes place likewise with reference to the polarity (see Chap. II); and the location of the primary germ-layers is determined by the polarity.

Not only is the ovum heteropolar, but in many bilateral animals, and perhaps in all, it is bilaterally symmetrical before cleavage begins; that is to say, one of the meridional planes defines the longitudinal axis of the future embryo, and the direction of anterior and posterior ends is also predetermined in this meridian, so that halves of the egg corresponding to future right and left sides of the embryo may be distinguished. In the frog's egg the plane of symmetry is marked by a gray crescent that appears above the equator on the side of the egg that corresponds to the hinder end of the embryo. This crescent is bisected by the meridional plane of symmetry. In the hen's egg the plane of symmetry of the embryo appears on the surface of the yolk in a line at right angles to the axis of the shell, and the left side of the embryo is turned towards the broad end, the right side towards the narrow end of the shell. The same plane of symmetry must exist in the ovum prior to cleavage for reasons explained beyond, although there is no morphological differentiation in the ovum proper, i.e., the germinal disc or yolk, that indicates it.

This predelineation of embryonic axes within the unsegmented ovum has been interpreted physiologically as due to gradients in rate of metabolic processes along the embryonic axes (Child), which determine the locaUzation of the main developmental events.